it depends.
mostly I send my Mac and my Netbook into standby and shut off the other machines.
unless one of my VMs on my vmhost has some work to do overnight then i leave it running.
it varies on what im doing, it makes my room warm so if its cold i leave it on, hot i turn it off, downloading is not a factor as my internet speed allows me to torrent all day without interrupting my other activities. usually i just leave it idle...
Technician says:
Shut down, remove all power.
No matter what the arguments are, here are some simple facts.
All electronics and mechanical hardware, hard drives, PSU's, fans... all have a life of a certain may hours of life, if left on those hour...